Puzzling structure: astronomers have discovered a gigantic gas filament on the underside of the Milky Way. The 3,900 light year long band of hydrogen is one of the largest known gas formations in our galaxy. The entire structure could contain around 1.7 million solar masses of gas, as the researchers report. Why this gas band lies 1,600 light years below the galaxy level and how it was formed remains unexplained for the time being.
At first glance, our Milky Way is a perfectly normal bar galaxy – a spiral galaxy with a central bar and four large spiral arms. All of these structures and a large part of the 100 to 300 billion stars lie in the flat, disk-shaped main plane of the galaxy. But in the last few years astronomers have discovered other, sometimes puzzling, structures in our galactic homeland. Among them are a huge wave winding through the main plane, huge bubbles of gamma rays and numerous stars and gas streams of extragalactic origin.
Hidden under the plane of the Milky Way
Now another structure has been added – a long band of hydrogen gas that runs under the main plane of our galaxy, but still lies parallel to it. “We don’t yet know exactly how it got there. But the filament runs about 1,600 light years below the plane of the Milky Way, ”reports first author Jonas Syed from the Max Planck Institute for Astronomy in Heidelberg. Astronomers discovered the first evidence of this “Maggie” structure around a year ago, but it remained unclear how big it is and what it is made of.
To find out more, Syed and his team analyzed images and spectral data from the Very Large Array (VLA) of the Green Bank radio telescope in the USA. In this data, they searched specifically for the radiation signature of neutral hydrogen gas, which typically emits radio radiation with a wavelength of around 21 centimeters. “The observations also made it possible to determine the speed of the hydrogen gas,” explains Syed’s colleague Henrik Beuther.
Gigantic filament made of gas
The analyzes revealed: The “Maggie” filament consists of atomic hydrogen gas and actually forms a single huge ribbon. This is about 3,900 light years long and 130 light years wide. “We have thus identified one of the largest, coherent filaments in the Milky Way, consisting largely of atomic hydrogen,” report the astronomers. In contrast, the largest known clouds of molecular gas are typically only around 800 light years in size.
“Maggie was already recognizable in earlier evaluations of the data. But only the current investigation proves without a doubt that it is a coherent structure, ”explains co-author Juan Soler from the MPI for Astronomy. In addition, the filament is about 55,000 light-years away from us on the other side of the Milky Way and does not contain any active star-forming regions. This makes it difficult to make out from Earth.
Where Maggie comes from and how this huge gas band came about remains a mystery for the time being. There are other large-scale structures in our galaxy, but their location outside the Milky Way plane and yet parallel to it is difficult to explain using common processes. Gas flows from supernova events usually have a random orientation, and even with gases flowing in from the outside, the chance of such a parallel orientation is minimal, as the team explains.
It would be conceivable, however, that “Maggie” only later reached its current position below the galactic plane: “We suspect that this filament was created in the middle of the plane and was then brought into this great distance by vertical oscillations of the galaxy,” write Syed and his colleagues.
© NASA/JPL-Caltech, ESO/R. Hurt & ESA/Gaia/DPAC, CC BY-SA 3.0 IGO & T. Müller/J. Syed/MPIA
Transition to the molecular star cradle?
The “Maggie” filament is unusual for another reason, however: the measurement data indicate that the atomic hydrogen gas converges at some points in this band, where it collects and condenses to form larger clouds. There the atomic gas could partially change into a molecular form. So far, however, the proportion of molecular hydrogen in this filament has only been around eight percent.
According to astronomers, “Maggie” could thus represent a link between atomic hydrogen flows and molecular clouds – the raw material from which stars are formed. “Maggie could then be the first representative of a class of atomic clouds that form the precursors of the gigantic molecular filaments,” explain Syed and his colleagues. According to this, new stars could one day be formed in this band of gas lying on the other side of the galactic disk.
“However, there are still many questions unanswered,” says Syed. “Further data, which we hope to provide more information about the proportion of the molecular gas, are already waiting to be evaluated.” (Astronomy & Astrophysics, 2021; doi: 10.1051/0004-6361/202141265)
Source: Max Planck Institute for Astronomy